Parallel operation of transformers



Sept. 23, 1958 H. E. PINNEY PARALLEL OPERATION OF TRANSFORMERS Filed May18, 1956 [II/677757 Harold P/Ime Qg 6W/Q 72% United States PatentPARALLEL OPERATION OF TRANSFORMERS Harold E. Pinney, Pittsfield, Mass.,assignor to General Electric Company, a corporation of New YorkApplication May 18, 1956, Serial No. 585,667

11 Claims. (Cl. 30724) This invention relates to automatic paralleloperation of alternating current power supply systems, and more inparticular to means for interconnecting a plurality of independentalternating current power supply systems such as voltage regulatingtransformer systems or other voltage regulating systems.

When two or more automatically controlled voltage regulatingtransformers such as load tap changing trans- Y formers are parallelconnected, circulating currents often flow in the circuit clue tounequal secondary voltages of the transformers. These circulatingcurrents do no useful work, and cause wasteful losses in the system.Various interconnecting circuits have been employed for correcting thiscondition automatically to reduce or entirely eliminate the circulatingcurrents. Typical circuits of this type are disclosed in U. S. PatentNo. 2,322,249 issued on June 22, 1943 on an application of S. Minneciand assigned to the present assignee and also in U. S. Patent No.2,323,716 issued on July 6, 1943 on an application of T. C. Lennox andalso assigned to the present assignee.

When regulating transformers are interconnected under load conditions,it is desirable that the voltages of the individual transformers besubstantially equalized prior to the interconnection in order to avoidexcessive momentary circulating current flow. Previously, equalizingmeans have been provided for the case where the transformers have acommon load, the provision of such means being disclosed in U. S. PatentNo. 2,363,891, which issued on November 28, 1944 on an application of S.Minneci assigned to the present assignee. In this type of equalizingsystem, the voltage of any idle unit that is to be put in service may bereadily equalized with the common load voltage, since' the line dropcompensator of the idle unit may be previously set at the samecompensation value as the units which are in service.

A different problem arises, however, in the case where severalregulating transformers having independent load circuits are to beinterconnected. In this case, the settings of the line drop compensatorsof the individual units may be diiferent, and thereby directinterconnection of the circuits would result in an unequal currentdistribution between the several interconnected units. The line dropcompensators of the power circuits are set to maintain constantpredetermined voltages at certain points (load centers) of theindependent load circuits, and the load centers of the power systems aregenerally at different locations so that the direct interconnection ofload centers is not feasible, and therefore it has been found to benecessary to equalize the voltages of the systems at points other thanthe load centers.

It is therefore an object of this invention to provide a control systemfor interconnecting electrical power circuits having individualautomatic voltage regulators and independent load circuits.

A further object of this invention is to provide means for equalizingthe voltages of a number of electrical power circuits having individualautomatic voltage regulators and independent load circuits, theequalizing ice means enabling the interconnection of the power circuitswith the desired division of load current between the systems andwithout momentary excessive circulating currents.

Briefly stated, in accordance with one aspect .of my invention, 1provide a system for interconnecting a plurality of electrical powercircuits, such as voltage regulating transformer circuits, havingseparate independent load circuits. Each of the electrical powercircuits has a separate automatic voltage regulating means connected tobe responsive to the voltage of its respective power circuit. Circuitmaking and breaking means are provided for selectively interconnectingthe load circuits of the power circuits. Each of the automatic voltageregulating means has a first line drop compensating means set forindependent operation of the respective power circuits, and the secondline drop compensating means set for interconnected operation of thepower circuits. Means are provided for interconnecting the regulatingcircuits, the interconnecting means preferably being relays. The relaysare operatively connected to disconnect the first line drop compensatingmeans from their respective automatic voltage regulating means,connecting the second line drop compensating means to the respectivevoltage regulating means, and for energizing means for providing apredetermined ratio of currents in the second line drop compensatingmeans. Time delay means are operatively connected to energize thecircuit making and breaking means for interconnecting the load circuitsa predetermined time after the second line drop compensating means havebeen connected to their respective automatic voltage regulating means.

While the specification concludes with claims particularly pointing outand distinctly claiming the subject mat ter which I regard as myinvention, it is believed that the invention will be better understoodfrom the following description taken in connection with the accompanyingdrawing.

The single figure of the drawing is a circuit diagram illustrating thepreferred embodiment of my invention as it may be applied to theinterconnection of two voltage regulating transformer circuits.

Referring now to the drawing, therein is illustrated a regulatingtransformer system comprising two power circuits (and their associatedvoltage regulating and control equipment) 10 and 10'. Since thesecircuits are substantially identical, in order to reduce confusion, thesame reference numerals will be employed to denote similar elements inthe two circuits, the elements of the circuit 10' being distinguished byhaving prime marks associated therewith. Accordingly, where the circuitsare identical, the detailed description will refer to only one of thecircuits.

The two power circuits 10 and 10' are supplied by a common source ofsupply 11 across which are connected the primary windings of theregulating transformers 12 and 12'. The two power circuits have separateload circuits 13 and 13 which are connected across the secondarywindings of the regulating transformers by pairs of leads 14 and 14 and15 and 15. The leads 15 and 15' are connected to the tapped ends of thesecondary windings of the regulating transformers, and the taps of thesecondary windings are changed by means of tap changing control systems16 and 16' of any suitable type, such as reversible motors 17 and 17'mechanically coupled to the tap changing switches of the regulatingtransformers.

A potential proportional to the output voltage of the regulatingtransformer 12 is provided by means of potential transformer 18 havingits primary winding connected across the leads 14 and 15. The secondarywinding of the potential transformer 18 is serially connected with thecoil 19 of voltage regulating relay 20 and line drop compensatingnetwork 21. The contact 22 of the voltage regulating relay 2% areconnected so that the tap changing motor control 16 is actuated to maketap changes on the regulating transformer to maintain a substantiallyconstant voltage across the coil 19 of the voltage regulating relay 2t).

In order to compensate for line voltage drop and thus provide a constantload voltage, a current transformer 23, coupled to the lead 15 andproviding a current proportional to the regulating transformer current,is serially connected with a portion of the line drop compensatorcircuit 21 as will be described in more detail in the followingparagraphs.

The line drop compensator circuit 21 is comprised of a reactor 24 havingwindings 25, 26, 27 and a reactor 28 having windings 29 and 30. Thewinding 26 and 29 are connected on one end by way of normally opencontacts 31 and normally closed contacts 32 respectively of relay 33 tothe coil 19 of voltage regulating relay 2G. The other ends of thewindings 26 and 29 are connected by way of potentiometers 35 and 36respectively to the secondary winding of the potential transformer 13.One end of the winding 27 is connected to one side of the currenttransformer 23, and the other ends of the windings 27 and 27' areconnected together by way of nor mally open contacts 37 of a time delayrelay 38. The end of the current transformer 23 connected to the winding27 is also connected by way of a primary winding load currenttransformer 40 and compensating current transformer 41 to a variable armon the winding 25. The end of the winding is connected by way of theprimary winding of an isolating transformer 42 to a variable arm on thewinding 33. The end of the winding is connected to the arm ofpotentiometer 36. The secondary winding of the isolating transformer 42is connected between the arm and one end of potentiometer 35. The otherend of the current transformer 23 is connected to the junction betweenthe secondary winding of the potential transformer 18 and thepotentiometers and 36.

The junction between the primary windings of the load currenttransformer 49 and compensating current transformer 41 is connected byway of normally open contacts d5 of relay 33 to the junction between theprimary windings of load current transformer 4t) and compensatingcurrent transformer d-lt'. One end of. each of the secondary windings ofload current transformers 4t} and 4 3' and compensating currenttransformers 61 and 4-1 are joined together. The secondary windings ofthe compensating current transformers 41 and 41 are short circuited bynormally closed contacts 46 of relay 33, and the secondary windings ofload current transformers it? and are short circuited by normally closedcontacts 47 of time delay relay 33.

A circuit breaker St) is provided having contacts 51 joining the leads14- and 14' and contact 52 joining the leads 15 and 15'. A circuitbreaker opening circuit 53 mechanically connected to open the contactsof the circuit breaker 5G is energized by way of normally closedcontacts 54 of relay 33. A circuit 55 for closing the contacts ofcircuit breaker 50 is energized by way of normally open contacts 56 oftime delay relay 38. The coil 57 of relay 33 is connected by way of anormally open push switch 59 to a source of electrical power 60. Holdingcontacts 61 are provided on the relay 33 for maintaining energization ofthe coil 57 in the conventional manner and for initiating theenergization of a coil as of time delay relay 38. A normally closed pushswitch 63 is provided in circuit interrupting arrangement in thecircuits of the windings 57 and 62 of relays 33 and 38 respectively.

When the two power circuits operate independently of each other, i. e.,when the contacts of the circuit breaker H formers 50 are open, the coil29 of reactor 28 is connected to the coil 19 of voltage regulating relay20 by way of contacts 32 of relay 33, the secondary windings of the loadcurrent transformers 4t) and the compensating current transformer 41 areshort circuited so that their primary windings present very lowimpedance to the flow of current, and the connection between thejunctions of primary windings of transformers 4t) and 41, and 40 and 41'is open. The connection between the windings 27 and 27' of the line dropcompensating circuit reactors is also open. The winding 25 andpotentiometer 36 may be set for the desired line drop compensation ofthe independent circuit 13. It is obviously not necessary that the linedrop compensation be the same for the two load circuits 13 and 13'.

When it is desired to interconnect the two systems, do pressing of theswitch 59 energizes the coil 57 of relay 32, and coil 57 remainsenergized by way of contacts 61.

Upon energizing of the relay 33, contacts 45 establish a connectionbetween the junction of the primary windings of transformers 4t) and 41and the junction of the primary windings of transformers 4t) and 41'.The contacts 46 are open to remove the short circuits on the secondarywindings of the compensating current trans- 41 and 41. Normally opencontacts 31 establish a connection between the winding 26 of reactor 24and the coil 19 of voltage regulating relay 20, and opening of thecontacts 31 removes the connection between the winding 29 of reactor 28and the coil 19 of voltage regulating relay 20. Similarly closing ofcontacts 31' establishes connection between the winding 26 of reactor24' and the coil 19 of voltage regulating relay 20 and opening ofcontacts 32 removes connection between the winding 29 of reactor 28 andthe coil 19 of the voltage regulating relay 20. The contacts 54 of relay33 open to remove power from the circuit breaker opening circuit 53, butthe circuit breaker does not close at this time.

The windings 25 and 25' and the otentiometers 35 and 35 are set so thatthe same line drop compensation is provided for each of the loadcircuits 13 and 13' respectively. Due to the connection of thecompensating current transformers 41 and 41' and the connectionestablished between the junctions of the primary windings of the loadcurrent transformers and compensating current transformers, the samecurrent or a predetermined ratio of currents is caused to flow in theline drop compensators of the two power circuits, and the tap changingcontrol systems of the two power circuits will be energized to maintainsubstantially the same output voltage. If the currents in the line dropcompensators are initially not equal, or of a predetermined ratio, aportion of the current of the circuit having excess current is forced byaction of the compensating current transformers to flow through the leadjoining the junctions of the load current transformers and circulatingcurrent transformers. The arrows illustrated on the drawing indicateinstantaneous current flow in and between the systems at this time ifthe current flowing in the system 10' is excessive.

After a predetermined period of time, the time delay relay 38 closes itscontacts 56 to energize the circuit breaker closing circuit 53, opensthe contacts 47 to remove the short circuit of the secondary windings ofthe load compensating transformers 40 and 4i), and closes the contacts37 to establish a connection between the ends of reactor windings 27 and27. The contacts 56 are preferably actuated before the contacts 47 and3'7, so that the circuit breaker 53- establishes connection between theload circuits 13 and 13 prior to actuation of contacts 37 and 47.

After the contacts of the circuit breaker have closed, the contacts 37and 47 complete the setting up of the interconnected circuits in themanner disclosed in the above-mentioned Minneci and Lennox patents. Theremoving of the short circuit on the secondary windings formers in adelta or open delta connection.

of the load current transformers 40 and 40' prevents circulating currentfrom passing through the reactor windings 2 5 and 25' in order toprevent erroneous tap changes resulting from circulating current, andthe establishing of the connection between the reactor windings 27 and27 completes a path for circulating current so that circulating currentflOWing through thewindings 27 and 27' provides the correct compensatingeffect in the event of unbalance of voltages of the two regulatingtransformers.

Theclosing of the contacts 45 effectively establishes a parallelconnection between the serially connected primary winding of transformer41, winding 25, winding 30, and potentiometer 36 and the seriallyconnected primary winding of transformer 41, winding 25, winding 3%, andpotentiometer 36.

The operation of the interconnecting system of my invention, stated inanother manner, provides voltage regulating means for the power circuitsthat are arranged to maintain constant predetermined voltages at theload centers of the different load circuits when the systems areindependently connected, but are arranged to establish substantially thesame voltage at the points of the load circuits to be interconnectedprior to the actual interconnection of the load circuits. After thecircuits are interconnected the line drop compensators serve to maintaina constant predetermined voltage at some point in each of the loadcircuits, although this point may not be the same as the load centersfor independent operation.

While the previous disclosure and drawing are directedv to a parallelconnection between the load circuits of a pair of voltage regulatingtransformers, it will be obvious that only slight modification isnecessary in order to apply the interconnected system to three or morepower circuits, and also that the interconnection need not be parallelconnections. For example, the same arrangement may be employed tointerconnect the trans- It is obvious that thevoltage regulatingtransformers may be of many different types, such as load tap changingpower transformers, auto-transformers, and induction voltage regulators.The power circuits may also be supplied by separate power sources.

It is also obvious that the line drop compensators for each powercircuit be combined on the same core with minor changes in the switchingcircuits.

Itwill be understood, of course, that, while the form of the inventionherein shown and described constitutes a preferred embodiment of myinvention, it is not intendedherein to illustrate all of the possibleequivalent forms or ramifications thereof. It will also be understoodthat the words employed are words of description rather than oflimitation, and that various changes may be made without departing fromthe spirit or scope of the invention herein disclosed, and it is aimedin the appended claims to cover all such changes as fall within the truespirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates'is:

1. A system for interconnecting a plurality of electrical power circuitshaving separate independent load circuits, said system comprisingseparate automatic voltage regulating means connected to each of saidpower circuits and continually responsive to the voltage of theirrespective circuits, circuit making and breaking means connected betweensaid load circuits for selectively interconnecting said load circuits,each of said automatic voltage regulating means having first line dropcompensating means for independent operation of said load circuits andsecond line drop compensator means for interconnected operation of saidload circuits, and means for interconnecting said automatic voltageregulating means for intercom nected operation of said load circuits.

- 2. A'system for interconnecting a plurality of electrical powercircuits having separate independent load circuits, said systemcomprising separate automatic voltage regulating means connected to eachof said power circuits and continually responsive to the voltage oftheir respective circuits, circuit making and breaking means connectedbetween said load circuits for selectively interconnecting said loadcircuits, each of said automatic voltage regulating means having firstline drop compensating means for independent operation of said loadcircuits and second line drop compensating means for interconnectedoperation of said load circuits, and means for interconnecting saidautomatic voltage regulating means comprising means providing apredetermined ratio of current in said second line drop compensatingmeans and time delay means for energizing said circuit making andbreaking means to interconnect said load circuits.

3. A system for interconnecting a plurality of electrical power circuitshaving separate independent load circuits, said system comprisingseparate automatic voltage regulating means connected to each of saidpower circuits and continually responsive to the voltage of theirrespective power circuits, circuit making and breaking means connectedbetween said load circuits for selectively interconnecting said loadcircuits, each of said automatic voltage regulating means having firstline drop compensating means for independent operation of said loadcircuits and second line drop compensating means for interconnectiveoperation of said load circuits, and timing means for interconnectingsaid automatic voltage regulating means for a predetermined time priorto the closing of said circuit making and breaking means.

4. A system for interconnecting a plurality of electrical power circuitshaving separate independent load circuits, said system comprisingseparate automatic voltage regulating means connected to each of saidpower circuits and continually responsive to the voltage of theirrespective power circuits, circuit making and breaking means connectedbetween said load circuits for selectively interconnecting saidload-circuits, each of said voltage regulating means having alternatelyconnected first and second line drop compensating means, circuit makingand breaking means for selectively interconnecting said automaticvoltage regulating means comprising means for connecting said secondline drop compensating means in their respective voltage regulatingmeans, means for providing a predetermined ratio of currents through thesecond line drop compensating means, and time delay means operativelyconnected to energize said circuit making and breaking means forinterconnecting said power circuits a predetermined time after saidconnecting means has connected said second line drop compensating meansin their respective voltage regulating means.

5. A system for interconnecting a plurality of electrical power circuitshaving separate independent load circuits,-said system comprisingseparate automatic voltage regulating means connected to each of saidpower circuits and responsive to the voltage of their respective powercircuits, circuit making and breaking means connected between said loadcircuits for selectively interconnecting said power circuits, each ofsaid voltage regulating means having first and second line dropcompensating means connected thereto alternately during independent andinterconnected operation of said power circuits respectively, and meansfor interconnecting said automatic voltage regulating means comprisingmeans for removing said first line drop compensating means andconnecting said second line drop compenssting means to their respectivevoltage regulating means, means for providing a predetermined ratio ofcurrent through said second line drop compensating means, and time delaymeans operatively connected to energize said circuit making and breakingmeans to interconnect said load circuits a predetermined time after saidsecond line drop compensating means have been connected to theirrespective voltage regulating means.

6. A system for interconnecting a plurality of electrical power circuitshaving separate independent load circuits, said system comprisingseparate automatic voltage regulating means connected to each of saidpower circuits and responsive to the voltage of their respective powercircuits, circuit making and breaking means connected between said loadcircuits for selectively interconnecting said power circuits, each ofsaid voltage regulating means having first and second line dropcompensating means connected thereto alternately during independent andinterconnected operation of said power circuits respectively, each ofsaid voltage regulating means having transformer means having firstwindings serially connected with the respective line drop compensatingmeans and second windings arranged to be short circuited duringindependent operation of said power circuits, and means forinterconnecting said automatic voltage regulating means comprising meansfor removing said first line drop compensating means and connecting saidsecond line drop compensating means to their respective voltageregulating means, means for removing the short circuits on said secondwindings and serially connecting said second windings, and time delaymeans operatively connected to energize said circuit making and breakingmeans to interconnect said load circuits a predetermined time after saidsecond line drop compensating means have been connected to theirrespective voltage regulating means.

7. A system for interconnecting a plurality of electrical power circuitshaving separate independent load circuits, said system comprisingseparate automatic voltage regulating means connected to each of saidpower circuits and responsive to the voltage of their respective powercircuits, circuit making and breaking means connected between said loadcircuits for selectively interconnecting said power circuits, each ofsaid voltage regulating means having first and second line dropcompensating means connected thereto alternately during independent andinterconnected operation of said power circuits respectively, each ofsaid voltage regulating means having transformer means having firstwindings serially connected with the respective line drop compensatingmeans and second windings arranged to be short circuited duringindependent operation of said power circuits, and means forinterconnecting said automatic voltage regulating means comprising meansfor removing said first line drop compensating means and connecting saidsecond line drop compensating means to their respective regulatingmeans, means for providing a predetermined ratio of currents betweensaid second line drop compensating means comprising means for removingthe short circuits on said second windings, serially connecting saidsecond windings, and parallel connecting said serially connected secondline drop compensation means and first windings, and time delay meansoperatively connected to energize said circuit making and breaking meansto interconnect said load circuits a predetermined time after saidsecond line drop compensating means have been connected to theirrespective voltage regulating means.

8. A system for interconnecting a plurality of regulating transformercircuits having separate load circuits, said system comprising separateautomatic voltage regulating means connected to each of said transformercircuits and responsive to the voltage of their respective transformercircuits, circuit making and breaking means connected between said loadcircuits for selectively interconnecting said load circuits, each ofsaid voltage regulating means having first and second line dropcompensating means connected thereto alternately during independent andinterconnected operation respectively of said transformer circuits, andmeans for interconnecting said automatic voltage regulating meanscomprising means for removing said first line drop compensating meansand connecting said second line drop compensation means to theirrespective voltage regulating means, means for providing a predeterminedratio of current through said second line drop compensating means, andtime delay means operatively connected to energize said circuit makingand breaking means to interconnect said load circuits a predeterminedtime after said second line drop compensating means have been connectedto their respective voltage regulating means.

9. A system for interconnecting a plurality of regulating transformercircuits having separate load circuits, said system comprising separateautomatic voltage regulating means connected to each of said transformercircuits and responsive to the voltage of their respective transformercircuits, circuit making and breaking means connected between said loadcircuits for selectively interconnecting said load circuits, each ofsaid voltage regulating means having first and second line dropcompensating means connected thereto alternately during independent andinterconnected operation respectively of said transformer circuits, andmeans for interconnecting said automatic voltage regulating meanscomprising first relay means operatively connected to disconnect saidfirst line drop compensating means from their respective voltageregulating means, to connect said second line drop compensating means totheir respective voltage regulating means, and to operatively connectmeans for providing a predetermined ratio of current through said secondline drop compensating means, and time delay means connected to energizesaid circuit making and breaking means to interconnect said loadcircuits a predetermined time after said second line drop compensatingmeans have been connected to their respective voltage regulating means.

10. A system for interconnecting a plurality of electrical powercircuits, said system having independent load circuits comprisingseparate automatic voltage regulating means connected to each of saidpower circuits and continually responsive to the voltages of theirrespective power circuits, circuit making and breaking means connectedbetween said load circuits for selectively interconnecting firstpredetermined points of said load circuits, and second predeterminedpoints in said lead circuits, each of said voltage regulating meanshaving line drop compensating means arranged to maintain firstpredetermined voltages at said second points when said lead circuits areindependently operated and to establish second substantially the samevoltages at said second points prior to the interconnection of said loadcircuits.

11. A system for interconnecting a plurality of electrical powercircuits, said system having independent load circuits comprisingseparate automatic voltage regulating means connected to each of saidpower circuits and continually responsive to the voltages of theirrespective power circuits, circuit making and breaking means connectedbetween said load circuits for selectively interconnecting firstpredetermined points of said load circuits, first line drop compensatingmeans in said voltage regulating means to maintain constant firstpredetermined voltages at second predetermined points in said loadcircuits when said load circuits are independently operated, second linedrop compensating means in said voltage regulating means to establishsubstantially the same voltage at said first predetermined points, andmeans to sclectively connect said line drop compensating means in theirrespective voltage regulating means.

References Cited in the file of this patent UNITED STATES PATENTS1,743,771 Hall Jan. 14, 1930 2,128,802 Crary Aug. 30, 1938 2,322,249Minneci June 22, 1943 2,447,644 Frisch Aug. 24, 1948 2,753,467 PinneyJuly 3, 1956

